Consolidate mutator contexts

Move most of the mutator context methods into BaseModuleContext
so they are available to both top down and bottom up mutators.
Only CreateModule is unique to TopDownMutatorContext, and the
dependency and variation adding methods are unique to the
BottomUpMutatorContext.

The dependency visiting methods are now available on
BottomUpMutatorContext, which requires delaying making newly
added dependencies visible to maintain the invariant that
the mutator has been called on the dependency before the
dependency can be visited by its parents.

Test: m checkbuild
Change-Id: Ie14afc02ac76d0b5a66b0e52de2aa9e17fd1bec0